The primary tanning of diverse skin material can be accomplished with different tanning agents, and the leathers inevitably take on an anionic or cationic charge character which depends on the charge of the tanning agent used. The tanning agents predominantly used are of the mineral type, such as, for example, basic chromium sulfates, which initially produce a cationic leather. However, a primary tanning alone is only very rarely sufficient for producing a certain required leather quality.
This explains the need for further supplemental tanning procedures, generally referred to as retanning, with which very specific visual and/or other sensory properties are obtained. The retanning agents used for this purpose are predominantly of an anionic nature and they recharge the leather, which is cationically charged from the primary tanning, more or less anionically. After these tanning stages have been concluded to produce the predetermined leather properties for which they are designed, the majority of such leathers are additionally dyed in the wet state in the drum. In particular, for the preparation of high-quality products, such as, for example, the so-called aniline leathers, it is of the utmost importance for the subsequent process of dyeing to produce visually flawless, uniform dyeings and not to cause any changes in the leather properties whatsoever due to any dyeing adjuvants used. For that reason there has understandably been no shortage of attempts to develop dyeing adjuvants which can meet these two main requirenents, including costly syntheses of substances of apparent suitability.
Since most leather dyes are anionic, the dye is uniformly distributed in the dyeing of anionic leathers, because the like charge prevents an excessively rapid and hence nonuniform strike of the dye on the leather surface. At the same time, however, deeper penetration of the dye into the interior of the fine or extremely fine fibers is also effected, as a result of which the visible leather surface appears to be short of color, i.e., paler.
The dyeings are therefore in general relatively uniform, but not sufficiently deep. This could be partially compensated by using more dye, but dyes are very costly and the bonding sites for dyes are limited. Additional bonding sites in the leather can be created by acidifying, and thus cationizing, the amphoteric leather system. This is generally done by adding acid, for certain reasons preferably formic acid. In addition to the fact that even in this way the necessary dye quantities cannot be satisfactorily fixed, a considerable disadvantage results in particular in the case of dyeing chrome tanned leathers, namely, that formic acid redissolves chrome constituents out of the leather and thereby impermissibly pollutes the liquors and waste waters with chromium.